The present invention relates to a semiconductor laser device, and in particular to a semiconductor laser device formed of InGaAlN-based materials.
InGaAlN-based materials are used for emitting blue to green light. Specifically, there have been attempts to apply a refractive index waveguide-type InGaAlN-based semiconductor laser device for an optical disk pickup since this type of semiconductor laser device emits light which is close to a plane wave and thus can reduce an astigmatic difference.
FIG. 4 shows a structure of a conventional refractive index waveguide-type blue semiconductor laser device 450. The conventional device 450 includes a sapphire substrate 400 and the following layers sequentially formed on the sapphire substrate 400: an n-type GaN contact layer 401, an n-type A0.1Gag0.9N cladding layer 402 having a thickness of 0.5 xcexcm, an n-type GaN optical guide layer 403 having a thickness of 100 nm, a multi-layer quantum well active layer 404 which includes three In0.2Ga0.8N quantum well layers each having a thickness of 2.5 nm and four In0.05Ga0.95N barrier layers each having a thickness of 3 nm, an Al0.1Ga0.9N protective layer 405 having a thickness of 25 nm, a p-type GaN optical guide layer 406 having a thickness of 50 nm, a p-type Al0.1Ga0.9N cladding layer 407 having a total thickness of 0.6 xcexcm which includes a lower flat region 410 having a thickness of 0.1 xcexcm and a ridge stripe portion 411 having a width of 2 xcexcm and a height of 0.5 xcexcm, and a p-type GaN contact layer 408 having a width of 2 xcexcm and a thickness of 0.2 xcexcm. The p-type GaN contact layer 408 is formed on the ridge stripe portion 411 of the p-type cladding layer 407. An SiO2 insulating layer 409 having a thickness of 0.3 xcexcm and a smaller refractive index than that of the ridge stripe portion 411 is formed so as to cover a top surface of the lower flat region 410 of the p-type cladding layer 407 and side surfaces of the ridge stripe portion 411. An n-type electrode 413 is formed on an exposed surface of the n-type GaN contact layer 401, and a p-type electrode 414 is formed on a surface of the p-type contact layer 408 on the ridge stripe portion 411. In the figure, reference numeral 412 represents ridge corners.
In the conventional semiconductor laser device 450, the refractive index of the SiO2 insulating layer 409 is smaller than the refractive index of the InGaAlN materials, and therefore the effective refractive index of the outside of the ridge is reduced, so that laser light is guided to the ridge region and the vicinity thereof.
The conventional InGaAlN-based semiconductor Ilaser device 450 was subjected to a reliability test under the conditions of 60xc2x0 C. and a constant output of 5 mW. The value of the operating current increased to 1.2 times or more the initial value, and the device 450 malfunctioned within 100 hours. Accordingly, the conventional semiconductor laser device is considered to have a life of about 100 hours. It was found that a life of 5000 hours or more, which is necessary for a semiconductor laser device used for an optical disk apparatus, cannot be realized.
The laser device before the reliability test and the laser device which malfunctioned during the reliability test were compared and analyzed. As a result, the post-malfunction laser device was observed to have crystal defects increased in the ridge corner areas 412 (both ends of the base of the ridge stripe portion 411 and the vicinity thereof). The present inventors found that in accordance with the increase in the crystal defects, the emission efficiency drastically declines in areas of the multi-layer quantum well active layer 404 which are contained in the ridge corner areas 412, and this is the main cause of reduction In the life of the conventional device 450. In the conventional device having the above-described structure, the crystal defect density of the post-malfunction ridge corner areas 412 was 6xc3x971011 to 8xc3x971011 cmxe2x88x922, which was higher than that of the defect density of the remaining area (3 to 7xc3x971010 cmxe2x88x922) by one order of magnitude.
The decline in the reliability due to the increase in the crystal defects is considered to be caused by the strong inter-atom bond of the InGaAlN-based crystal itself and by a large difference between the thermal expansion coefficient of the material forming the ridge stripe portion 411 (p-type Al0.1Ga0.9N cladding layer 407; 5.6xc3x9710xe2x88x926/xc2x0 C.) and the thermal expansion coefficient of the.material surrounding the bottom portion of the ridge stripe portion 411 (SiO2 insulating layer 409; 0.5xc3x9710xe2x88x926/xc2x0 C.), the difference being as large as +5.1xc3x9710xe2x88x926/xc2x0 C. In other words, when the conventional device 450 which has such a large difference in the thermal expansion coefficient between the two materials is made conductive, heat is generated In the ridge stripe portion 411 and the vicinity thereof where the current is concentrated, and thus the temperature is raised locally. It Is considered that the crystal defeat and crystal breakage occurred for the following reason. When the local temperature rise occurs in the InGaAlN-based crystal of the above-described structure, thermal distortion is induced at the ridge corner areas 412 by the difference in the thermal expansion coefficient between the p-type Al0.1Ga0.9N cladding layer 407 forming the ridge stripe portion 411 and the SiO2 insulating layer 409 surrounding the ridge stripe portion 411. The crystal defeat and crystal breakage started from the ridge corner areas 412. Such a phenomenon, which was not observed with InGaAlAs-based materials or InGaAlP-based materials which were conventionally laser used for ridge stripe-type laser devices, was specific to an InGaAlN-based (nitride-based) semiconductor device having a strong bond between a group III atom and a nitrogen atom.
A semiconductor laser device according to the present invention Is an InGaAlN-based semiconductor laser device including a first layer of a first conductivity type, an active layer having a smaller forbidden band than that of the first layer, and a second layer of a second conductivity type having a larger forbidden band than that of the active layer, wherein the second layer includes a flat region and a stripe-shaped projecting structure; a stripe-shaped optical waveguide forming layer of the second conductivity type having a larger refractive index than that of the second layer is formed on the stripe-shaped projecting structure; a current-constricting layer of the first conductivity type or of a high resistance is formed for covering a top surface of the flat region of the second layer, a side surface of the projecting structure of the second layer, and a side surface of the optical waveguide forming layer; and a difference between a thermal expansion coefficient of the current-constricting layer and a thermal expansion coefficient of the second layer is in the range of xe2x88x924xc3x9710xe2x88x929/xc2x0 C. to +4xc3x9710xe2x88x92/xc2x0 C.
Due to such a structure, even when the temperature of the device is locally increased in the stripe-shaped projecting structure and the vicinity thereof in which the current is injected in a concentrated manner, the thermal distortion of the device is suppressed owing to the small difference in the thermal expansion coefficient between the current-constricting layer and the second conductive layer. Therefore, a local crystal defect or crystal breakage is avoided, thus extending the life of the laser device.
In one embodiment of the invention, the second layer and the current-constricting layer are formed of an InGaAlN-based semiconductor material of the same composition. For example, the second layer may be formed of AlxGa1xe2x88x92xN, and the current-constricting layer may be formed of AlyGa1xe2x88x92yN, (xe2x88x920.08xe2x89xa6xxe2x88x92yxe2x89xa60.08). In an alternative embodiment of the present invention, the optical waveguide forming layer is formed of InuGa1xe2x88x92uN, and u is 0.02 or more and is 90% or less of an In-mix crystal ratio of a well layer included in the active layer.
Due to such a structure, th e thermal expansion coefficient which varies depending on the Al-mix crystal ratio, can be less different between the current-constricting layer and the second layer, and thus the thermal distortion of the device can be reduced. By forming the optical waveguide forming layer of InuGa1xe2x88x92uN, the refractive Index of the optical waveguide forming layer can be higher than that of the flat region of the second layer on which the current-constricting layer is formed. This way, a laser device stably oscillating in a single transverse mode up to a high output can be obtained. In addition, the wavefront of the output laser beam is substantially plane and the astigmatic difference can be reduced.
Furthermore in one embodiment of the present invention, the second layer or the current-constricting layer is formed of a super-lattice structure, and the second layer and the current-constricting layer have the same average mix crystal ratio. In one embodiment of the present invention, an InGaAlN layer included in the super-lattic e structure has a thickness of 50 nm or less.
Due to such a structure, the thermal expansion coefficients of the second conductive layer and the current-constricting layer are substantially the same, and thus the crystal deterioration at the ridge corners is suppressed. Therefore, the life of the laser device can be extended. By forming the layer having the super-lattice structure to have a thickness of 50 nm or less, the influence of the super-lattice layer on the ridge corner areas 112 can be considered based on the average composition of the super-lattice structure.